In a semiconductor device wherein a semiconductor chip is mounted on a lead frame and these lead frame and semiconductor chip are integrally sealed by a resin mold, dam bars connect leads of the lead frame together and serve to dam a mold resin from flowing out between the leads when the lead frame and the semiconductor chip are integrally sealed by the resin mold. The dam bars also serve to reinforce the leads. After the integral sealing by the resin mold, the dam bars are cut and removed so that the leads (outer leads) of the lead frame are separated from one another. While the semiconductor device wherein a semiconductor chip is mounted on a lead frame and these lead frame and semiconductor chip are integrally sealed by a resin mold is sometimes called a resin-molded semiconductor device, it will be referred to simply as a semiconductor device in the description below.
Heretofore, since the lead pitch of lead frames is not so fine and some margin is provided in dimensional accuracy of an outer lead portion, dam bars have been often punched and cut by using a punching press. With a recent increase in degree of integration and performance of semiconductor devices, however, lead frames have had even higher pin count and finer pitch, thus requiring more strict dimensional accuracy. Then, it has become difficult to achieve such strict dimensional accuracy by the conventional press punching method from the technical point of view.
For a high pin count, fine-pitch lead frame with the pitch of 0.3 mm, by way of example, a frame thickness is about 0.1 to 0.2 mm and a width of lead gaps (hereinafter referred to also as slits) is about 0.1 to 0.15 mm. This means that a dam bar portion requires to be cut with the comparable or finer dimensions to or than the frame thickness. Manufacturing a tool capable of punching lead frames with such fine dimensions entails a great difficulty. Even if such a tool is manufactured, it would be broken with much possibility because the tool must have a fine thickness. Further, since the resin (hereinafter referred to as within-dam resin) that has been dammed by the dam bars and the resin (hereinafter referred to as resin burrs) that have flown out over surfaces of the lead frame are deposited near the dam bars, the press punching method cannot positively cut the dam bars into satisfactory configurations.
Meanwhile, the method of cutting dam bars by utilizing a laser beam has been developed in recent years. Because a laser beam can be condensed to a very small spot suitable for fine processing, the dam bars can be cut in a non-contact manner with high dimensional accuracy just by irradiating the laser beam to the dam bars. The prior art in which such a cutting process utilizing a laser beam (hereinafter referred to also as laser cutting) is applied to cutting of dam bars is described in JP, A, 4-322454, for example.
In this prior art, the cross-section of a laser beam output from a laser oscillator is transformed into an oblong shape (elliptical shape) by a cylindrical lens, a focused position of the laser beam is determined by a galvano-mirror, and the laser beam is condensed by a large-aperture condensing lens to a dam bar for melting and cutting it. The dam bar is cut by one shot of the laser beam irradiated corresponding to a narrow slit width. (This prior art will be hereinafter referred to as first prior art).
Also, the prior art disclosed in JP, A, 5-211260 proposes a method comprising the steps of taking in cut positions of individual dam bars by a camera or the like beforehand, detecting actual positions of all the dam bars through image processing after taking in all data, and then irradiating a laser beam based on the correct position information detected. This method also employs a galvano-mirror for determining a focused position of the laser beam and a condensing lens of large aperture for condensing the laser beam. (This prior art will be hereinafter referred to as second prior art).
On the other hand, though not concerned with the dam bar cutting method, the prior art disclosed in JP, A, 1-224188 proposes a (on-the-fly) technique for relatively moving the optical axis of a laser beam along a cutting path determined beforehand, while controlling the oscillation time of the laser beam, and continuously cutting a workpiece while positioning the laser beam at high speed, for the purpose of avoiding vibration induced due to acceleration and deceleration when the workpiece is intermittently cut by the laser beam. This technique is suitable for cutting semiconductor wafers, but appears to be also applicable to cutting of dam bars. (This prior art will be hereinafter referred to as third prior art).
Further, JP, A, 6-142968 describes the prior art suitable for cutting dam bars by utilizing a laser beam. In this prior art, the presence or absence of materials in a workpiece (i.e., the array condition of leads) is optically detected by a detecting optical system (light detecting means) almost coaxially with a cutting optical system, the detection signal is changed into two value form, followed by giving a certain delay time to the square signal, and the oscillation of a laser beam is controlled based on the signal given with the delay time. In other words, this process is designed to directly visually detect the array condition of leads and to promptly irradiate a pulsed laser beam by utilizing the detected data. Thus, just by moving the optical axis of the laser beam (hereinafter referred to as laser beam axis) with respect to the workpiece, dam bars can be successively cut at high speed with certainty. (This prior art will be hereinafter referred to as fourth prior art).